June 30, 2014

A Polar Satellite Launch Vehicle blasted off from India’s Satish Dhawan Space Center on Monday morning at 4:22 UTC, embarking on a mission to deliver the SPOT-7 Earth Observation satellite and secondary payloads from Germany, Canada and Singapore to orbit. Making its second flight of the year, India’s workhorse launcher completed a flawless 20-minute ascent mission, delivering all five payloads to the planned orbit.

The SPOT-7 spacecraft is set to enter a constellation of Earth imaging satellites operated by by Airbus Defence and Space and SPOT Image, marketing high-resolution Earth imagery to commercial and government customers. CanX-4 and CanX-5 – the Canadian duo onboard this flight – will conduct a precise formation flying exercise in orbit testing new navigation and micropropulsion systems. AISat, a satellite of the German Aerospace Center, will demonstrate new technology for the Automatic Identification System, tracking sea vessels around the globe. Finally, the VELOX-1 satellite from Singapore will separate a small Nanosatellite to demonstrate inter-satellite communications along with an onboard physics payload.

Monday’s launch was preceded by a 49-hour countdown that got underway on Saturday morning to prepare the PSLV for liftoff by loading the launcher with propellants and completing final reconfigurations and checkouts of the vehicle. PSLV uses a combination of solid- and liquid-fueled stages – the PS1 first stage and PS3 third stage use solid propellants that are loaded ahead of launch vehicle integration. The PS4 stage and the Roll Control Thruster Tanks on PS1 were filled with hypergolics on Saturday while the second stage received its load of storable propellants on Sunday.

Photo: Indian Space Research Organization

Lifting the relatively small SPOT-7 satellite and the four secondary payloads to orbit allowed PSLV to launch in its most basic configuration, known as Core Alone that differs from the regular and XL version of the rocket by cutting the six Solid Rocket Boosters and launching the fourth stage with a partial fuel load. Overall, the C23 PSLV stood 44.5 meters tall, had a diameter of 2.8 meters and a liftoff mass of 229,000 Kilograms.

The rocket was exposed on its pad on Sunday when the Mobile Service Tower was moved back to its launch position, 160 meter from the pad, to clear the way for final close-outs of the PSLV and the launch facility.

Photo: Indian Space Research Organization

Throughout the countdown, teams were watching over all systems of the rocket and the payload and completed electrical tests, checkouts of the communications system, control system verifications and Flight Termination System testing. Flight computers were configured for the Terminal Countdown Sequence and the final systems check was performed less than one hour from launch. When all systems were verified in good condition, the formal authorization for launch was given and the countdown headed into its Terminal Sequence at T-8 minutes.

At that point, the main payload, SPOT-7, transitioned to its flight mode, also switching to internal power while the other satellites were already ready for launch with their sequencers timed to start up at the moment of separation from the launcher.

At T-5 minutes, the flight computers of PSLV were configured for flight and received their appropriate flight software.

Ground tracking station readiness was verified to ensure PSLV could be tracked on its way downrange. At T-3 minutes, the launch vehicle transitioned to flight mode. Final countdown procedures included the pressurization of the propellant tanks aboard the launch vehicle that also transferred to internal power. At T-1 minute, the on-board Master Sequencer assumed control of the countdown, putting the rocket through the final steps ahead of liftoff.

Three seconds before T-0, the Roll Control Thrusters of the first stage were ignited, burning hypergolic propellants for roll control during first stage flight. When clocks hit zero, the PS1 first stage was ignited and PSLV jumped off its pad – right on time at 4:22 UTC, 9:52 local time on India’s East Coast.

With a total launch thrust of 459,600 Kilogram-force, PSLV started racing uphill, initially rising vertically before making a pitch and roll maneuver to align itself with the planned launch trajectory.

As this mission was targeting a polar orbit, it included a Dogleg maneuver – a peculiarity of launches out of locations that have geographic restrictions in the form of inhabited areas in the vicinity of the ideal ascent path. For launches from SHAR heading to polar orbit, rockets have to make a powered turn during their ascent to avoid the island of Sri Lanka. This guided turn comes at the cost of launch vehicle performance, but is a requirement to keep life and property safe.

After heading south-east, PSLV started to turn to the south-west, headed for a 98-degree orbit. Powered by the first stage, PSLV quickly passed Mach 1 and Maximum Dynamic Pressure as the core burned through 1.2 metric tons of propellant per second. PS1 is one of the largest solid-fueled rocket stages ever built, measuring 20.34 meters in length and 2.8 meters in diameter. The PS1 launched with a fuel load of 138 metric tons to provide 495,600 Kilograms of thrust during a 105-second burn. Vehicle control was provided via Secondary Injection Thrust Vector Control for pitch & yaw and two thrusters for roll.

Thrust on the first stage tailed off at T+1 minute and 45 seconds, followed by separation five seconds later, at T+110.5 seconds. The first stage helped accelerate the vehicle to 1.55 Kilometers per second as it separated at an altitude of 53 Kilometers. 1.4 seconds after the pyrotechnic separation of the PS1 stage was triggered, the second stage ignited its large Vikas 4 engine for a 2-minute 32-second burn.

Photo: Indian Space Research Organization

80 seconds into the second stage burn,
the launch vehicle enabled Closed Loop Control. The first stage
flew a pre-determined attitude profile while the remaining stages
used navigation data to optimize the vehicle’s trajectory for an
accurate insertion

The second stage of the PSLV is 12.8 meters long and capable of holding 40,700 Kilograms of Unsymmetrical Dimethylhydrazine fuel and Nitrogen Tetroxide oxidizer. The Vikas 4 engine provides 81,500 Kilograms of thrust operating at a chamber pressure of 58.5 bar, consuming 278 Kilograms of propellant per second.

Three minutes and seven seconds after launch, PSLV was at an altitude of 132 Kilometers, well outside the dense atmosphere allowing the protective payload fairing to separate. Now exposed for the rest of their way into orbit, the five payloads enjoyed a nominal flight.

The second stage completed its burn at T+4:22, followed moments later by stage separation at an altitude of 220 Kilometers when PSLV was moving at 3.6 Kilometers per second. One second after PS2 separation, the PS3 stage ignited its solid rocket motor for a burn of 112 seconds. The PS3 stage has a reduced diameter of 2.02 meters, being 3.54 meters long. It is loaded with 6,700 Kilograms of HTPB based propellant. PS3 provided 24,900 Kilograms of thrust over the course of its burn – boosting the vehicle’s velocity by 1.8 Kilometers per second.

At T+6 minutes and 18 seconds, the third stage burned out and thrust ramped down. PSLV held onto its third stage for two and a half minutes to allow residual thrust to tail off to ensure a safe stage separation later as PSLV entered a coast phase. Stage separation took place at T+8 minutes and 46 seconds. Eleven seconds later, the fourth stage ignited its two engines as the vehicle passed an altitude of 546 Kilometers. Over the course of an eight-minute 26-second burn, the upper stage of the vehicle accelerated the stack from 5.33 to its insertion speed of 7.52 Kilometers per second.

In the PSLV CA configuration, the PS4 stage launches with a partial fuel load of 1,600 Kilograms of Monomethylhydrazine and Mixed Oxides of Nitrogen. PS4 is 2.02 meters in diameter and 2.6 meters long and its two L-2-5 engines provide 1,490 Kilogram-force of thrust.

Image: DD News Live

Fourth stage cutoff occurred at T+17
minutes and 13 seconds when the vehicle was orbiting at an
altitude of 659 Kilometers. The mission was targeting an orbit of
655.1 by 657.7 Kilometers at an inclination of 98.23 degrees for
the insertion of the five payloads.

PSLV once more provided an excellent insertion accuracy to its passengers, reaching the target orbit without significant inaccuracies. With the completion of the ascent sequence, the fourth stage initiated its re-orientation for the separation of the payloads.

Image: Airbus Defence & Space

Above: SPOT-7, Below: CanX-4 & 5

Photo: University of Toronto/SFL

SPOT-7 was sent on its way just before T+18 minutes, beginning a long mission as part of the SPOT & Pleiades Earth Observation satellite fleet set to deliver continued imagery of Earth for a variety of applications. The addition of SPOT-7 will significantly enhance revisit and response times of the constellation, enabling a rapid delivery of imagery.

AISat was released 37 seconds after SPOT-7, set for a demonstration mission of a new helical high-gain antenna system for the acquisition of Automatic Identification System messages from ships around the globe including highly frequented sea areas where conventional AIS terminals can not provide usable data due to signal overlap.

The Canadian duo, CanX-4 and 5 completed deployment at T+19:01 and T+19:31 being released from the launcher to find each other again in orbit to conduct a formation flying exercise with high precision as part of the demonstration of new navigation algorithms and a micro propulsion system.

The final satellite to be released was VELOX-1 that was sent into orbit at T+19:55. This 3U CubeSat will conduct a quantum physics experiment, demonstrate new satellite technology and separate a small NanoSat to test an inter-satellite communication system.

The PSLV C23 mission also carried an attached payload – the Advanced Inertial Navigation System. AINS is a new guidance unit to be used on future launch vehicles to deliver more accurate navigation while reducing the size and mass of the navigation system. During the mission, AINS was not used to navigate the PSLV launcher – data collected by the system was recorded and downlinked for the assessment of the performance and accuracy of the system.

Concluding its flight 20 minutes after liftoff, PSLV checks off another success. Two more PSLV missions are planned for this year, both carrying Indian navigation satellites in support of the deployment of the Indian Regional Navigation System. The next Indian launch will be the maiden mission of the Geostationary Launch Vehicle Mk. III set for late July or early August

Payload Overview

﻿SPOT-7﻿

SPOT-7 is a high-resolution wide-swath imaging spacecraft built and operated by Airbus Defence and Space taking over the majority of Spot Image after the government support of the SPOT program was terminated. SPOT-6 – launched in 2012 – and SPOT-7 are identical spacecraft, based on the AstroSat-250 satellite bus and use the NAOMI (New AstroSat Optical Modular Instrument) payload to acquire optical imagery to ensure the continuity of SPOT data, building on experience gained through previous missions, particularly SPOT-5 that launched in 2002.

SPOT satellite imagery is marketed to commercial customers for a variety of purposes and data is also combined with radar imagery acquired by other satellites to generate high-level data products. The imagery covers wide ground swaths at a very high resolution and the spacecraft and orbit design allows for rapid revisit times as low as one day. Data is provided with low latency and is available to customers shortly after acquisition and the satellite provides just-in-time tasking capability.

The SPOT-7 spacecraft is built for a ten-year mission featuring two NAOMI cameras to cover a 60-Kilometer ground swath, 120km using single-pass mosaic imaging. Overall, the satellite can achieve a resolution of two meters in panchromatic and eight meters in multispectral mode covering the visible and near-infrared spectral bands.

SPOT-7, based on AstroSat-250, measures 1.55 by 1.75 by 2.70 meters in size with a liftoff mass of 720 Kilograms. The Astrosat-250 platform uses a modular approach and provides a one-fault tolerant architecture across all subsystems. One of the most critical features of the satellite platform is the use of Control Moment Gyros that allow the satellite to make rapid attitude maneuvers for quick re-targeting

The AstroSat-250 bus is hexagonal in shape using composite panels and interior panels to provide mounting surfaces for the various satellite systems. The spacecraft is equipped with three deployable solar panels that are fixed in position and feature triple-junction Gallium-Arsenide solar cells. Overall, the solar panels provide a nominal power of 1,200 Watts using a shunt regulation system.

Image: Airbus Defence & Space

Image: Airbus Defence & Space

Optionally, a Maximum Power Point Tracker can be used to increase power output for payloads with higher power demand. Electrical power delivered by the arrays is passed to the Power Control and Distribution Unit that provides power regulation and distribution functions, conditioning an unregulated 28-Volt power bus and controlling the state of charge of Li-Ion batteries used for power storage.

Image: Airbus Defence & Space

Image: Airbus Defence & Space

Spacecraft Propulsion is accomplished using a Hydrazine monopropellant system. The PM22 system is used for rapid rate dampening in acquisition mode, in Earth-pointed safe mode and for regular orbit adjustment and maintenance maneuvers to keep the spacecraft orbiting close to its reference orbit. A series of thrusters are installed on two thruster pods that are supplied from a 104-liter propellant tank that operates in blow-down mode. The 1N thrusters thrusters can tolerate supply pressures of 5.5 to 23 bars to generate a thrust of 0.36 to 1.45 Newtons. The corresponding specific impulses are 205 seconds at the lowest supply pressure and 221 seconds at the highest pressure. Each thruster assembly weighs about 0.23 Kilograms and can be operated in steady-state mode and pulse mode for attitude control.

Attitude control is provided by four reaction wheels and three magnetic torquers are used for coarse control and for momentum dumps of the reaction wheels. Three-axis attitude and orbit measurement is accomplished through a Star Tracker Assembly with three optical heads and a GPS unit. The star tracker delivers pointing knowledge of up to 30 µrad while the attitude control system provides a pointing accuracy of 500 µrad. An inertial measurement unit is used to improve three-axis rate measurements and serves as backup to the star trackers. The dual-frequency GPS receiver can be used for orbit determination with an accuracy of 3 meters. In spacecraft safe mode, attitude determination is provided by a Magnetometer and Coarse Sun Sensor that are used to keep the satellite in a safe Earth- or Sun-pointed attitude.

Payload data is downlinked via a two-channel cold redundant X-Band system that achieves a data rate of up to 300Mbit/s using QPSK modulation. The communication system uses a single isoflux antenna. Telemetry downlink and command uplink is accomplished using an S-Band system with omni-directional coverage.

Command & data handling, spacecraft control and communication is provided by a control system built around the SCOC3 Spacecraft Controller On-a-Chip that uses a LEON-3 microprocessor.

The redundant onboard computer provides processing, reconfiguration and timing functions, data input/output and customization capabilities. A Remote Interface Unit allows the spacecraft computer to be reconfigured in flight. Two 1553 data buses are used to handle spacecraft platform data and data acquired by the payload. The flight software used by the spacecraft is based on the RTEMS operating system with customizable software elements tailored for specific mission purposes. The flight computer uses an autonomous Failure Detection, Isolation and Recovery system.

Data delivered by the satellite payload is stored by a Compression Recording and Ciphering unit, CoReCi. This system uses flash-based technology and a modular approach to provide an incremental data storage capacity up to 10Tbit. CoReCi supports input data rates up to 1.4 Gbit/s. The system uses embedded Wavelet Image Compression, data ciphering using Advanced Encryption Standard, and data formatting according to ESA’s packet-telemetry standard. The system weighs 14 Kilograms and has a peak power consumption of 75 Watts.

The main payload of the SPOT7 spacecraft consists of two NAOMI optical imagers. The New AstroSat Optical Modular Instrument has extensive flight heritage having flown on the KazEOSat-1, SPOT-6, SSOT and VNREDSat-1A satellites. The instrument weighs 150 Kilograms and has a peak power demand of 180 Watts.

The instrument is a high-resolution pushbroom-type imager that was developed by EADS Astrium and SAS. NAOMI is comprised of an optical bench consisting of SiC to provide extremely high thermal stability, a focal plane assembly with Time Delay Integration Detector, back-end electronics for data processing and interfaces for data and command exchange with the spacecraft computer.

The telescope uses a Korsch combination with three aspheric mirrors and two folding mirrors using an aperture diameter of 20 centimeters. This design was chosen because of its simplicity and compact size – fitting within the small spacecraft platform. An entrance baffle is used for stray light rejection. Light entering the detector is passed from the primary mirror M1 onto the M2 mirror before passing Folding Mirror 1 to the M3 mirror that reflects the light through the Exit Pupil to Folding Mirror 2 that passes the light onto the detector.

The Time Delay Integration detector uses a silicon CCD detector assembly with 7000 pixels for the panchromatic channel and four lines of 1750 pixels for the multispectral bands. All detectors are equipped with strip filters and front end electronics. The front end electronics provide the detectors with biasing and clocking signals as well as pre-amplification of the signals before transmitting them to the video electronics. The back-end electronics provide power supplies for the operation of the front-end and provide data processing via modular video chains that operate at frequencies of up to 15Msamples/sec. The signals from the front end is digitally converted in the back end using a 12-bit conversion scheme and the data is then transferred for real-time processing and storage in the mass memory. The instrument covers five bands – the panchromatic band of 450 to 750 nanometers and four multispectral bands including blue (450-520nm), green (530-600nm), red (620-690nm) and near infrared (760-890nm). The telescope covers a ground swath of 20 Kilometers and the spacecraft has a field of regard of +/-35 degrees (800km) as it is tilted around nadir for event monitoring. NAOMI on SPOT7 can deliver panchromatic images at a ground resolution of two meters at nadir while the multispectral imagery will reach resolutions of eight meters.

Image: Astrium/SAS

Image: Astrium/SAS

Detector Design & Spectral Coverage

Image: Astrium/SAS

For imaging sessions, SPOT-7 can use its agile attitude control system to support a variety of observation modes. The spacecraft is capable of acquiring a multitude of scenes in a localized area – up to 11 scenes – each 60 by 60 Kilometers - can be collected within a 1,000-Kilometer long ground strip. Strip imaging is also possible and SPOT-7 can acquire multiple strips of a target area in a single pass with strip lengths of up to 600 Kilometers. Mosaic strip-imaging in which the satellite acquires multiple image strips of adjacent areas is also possible to cover an 300 by 330-Kilometer rectangular target area in a single pass. For regular imaging, SPOT-7 will use off-nadir angles up to 35 degrees which also allows the spacecraft to support quick-revisit times of one day.

Image: Astrium/SAS

SPOT-7 can also collect stereo and tri-stereo images in one pass. Data is automatically geo-referenced using an automated system that operates at an accuracy of 10 to 20 meters after calibration.

SPOT-7 is planned to operate in a circular orbit at 695 Kilometers at an inclination of 98.2 degrees. The satellite will be phased into a constellation with the SPOT-5 and SPOT-6 satellites as well as the Pleiades 1A and 1B spacecraft to establish an Earth Observation Constellation with fast revisit times and quick response times to targets of opportunity.

SPOT-7 utilizes a simplified Ground Control Segment consisting of the CGS – the Control Ground Segment and the EGS – the Exploitation Ground Segment. The division of labor between these two components is very clear – the CGS is in charge of commanding the spacecraft, monitoring its health, performing periodic maintenance operations, planned orbital maneuvers and accepting the mission plan from the EGS. The EGS plans the targets that are to be imaged by the satellite and accepts all data downlinked by the spacecraft for processing, distribution to customers and archiving. SPOT-7 supports up to six mission plan updates per day allowing quick response times and just-in-time tasking with rapid delivery of data products after acquisition.

Image: Astrium/SAS

﻿CanX-4 & CanX-5﻿

Image: University of Toronto SFL

Image: University of Toronto SFL

The CanX-4 and 5 satellites (Canadian Advanced Nanospace eXperiment) were developed at the University of Toronto to conduct a demonstration mission dedicated to Satellite Formation Flying using high-accuracy tracking demonstrating algorithms for autonomous formation maintenance in the presence of orbital perturbations with sub-meter accuracy in Earth’s uneven gravitational field.

The CanX program was initiated to allow graduate students gain experience in the development, manufacturing and operation of satellite missions and to test out new, innovative technologies for future application in space missions. CanX-4 and 5 will demonstrate the autonomous and maintenance of a dual-satellite constellation using various geometries, a carrier-differential GPS measurement technique for high-precision relative position determination, and a nanosatellite propulsion system for use to maintain the formation.

The two satellites will perform a one-week formation flying campaign after performing a rendezvous to meet up following the release from the launch vehicle. The operation is limited in time due to the restricted supply of propellant. Once separated, the satellites will demonstrate formation flying at distances of 50 to 1,000 meters using a position control better than one meter and a relative position determination of 10 centimeters.

For the demonstration, the two spacecraft will use GPS information to calculate relative position. An inter-satellite communication system will allow the spacecraft to exchange GPS and attitude data for precise navigation using an accurate attitude control and nanopropulsion system.

CanX-4 and CanX-5 are identical satellites based on SFL’s Generic Nanosatellite Bus that provides all the required subsystems for the operation of a variety of payloads leaving about 30% of its total volume open for use by payloads. Using the same platform for several previous missions led to a quick build-up of flight heritage and performance data which is of great value when conducting experimental missions.

The satellite bus features a cubical design with a 20-centimeter side length using aluminum exterior panels and two internal trays to host the various satellite subsystems and create a payload bay for simple integration of satellite payloads of different kinds. Each CanX spacecraft weighs around 7 Kilograms.

Power is provided by four to ten triple-junction GaAs solar cells installed on each of the external panels delivering up to ten Watts of power using Peak Power Point Tracking provided by the Battery Charge/Discharge Units. Power is stored in two Li-Ion batteries with a capacity of 5.3 Ah. The power conditioning unit provides a 4-Volt unregulated power bus.

Attitude Determination is accomplished by a three-axis magnetometer, six sun sensors for fine and sun attitude determination and a star tracker for precise attitude determination. The Miniature Star Tracker provides three-axis attitude solutions at a control cycle at 0.5 Hz and an accuracy of 10arcsec. Attitude actuation is provided by three reaction wheels with a total mass of 185grams and a volume of 5 by 5 by 4 centimeters. The wheels have a momentum capacity of 30mNms and deliver a maximum torque of 2mNm. Momentum dumps are supported by three magnetotorquers.

Image: University of Toronto SFL

Data handling and satellite control is provided by an ARM7 housekeeping computer that handles standard telemetry and communications while a second computer supports all attitude determination and control functions. A third computer board is in charge of the operation of the science payload and handles its data. Each processor board uses the ARM7/TDMI processor with a code memory of 256kB and 2MB of hardware SRAM memory used to store program variables and data. A 256MB flash memory is used for long-term data storage.

The communications system of the satellites includes a UHF receiver, an S-Band Transceiver and a VHF beacon. The UHF receiver will be used for the command uplink from the ground at a data rate of 4kbit/s in the amateur radio band using quad-canted monopole antennas for omni-directional coverage. The VHF beacon transmits the satellite’s identification and some basic telemetry values for satellite tracking and the initial commissioning of the spacecraft.

The S-Band system is used for data downlink to the ground and the Intersatellite Communication System to allow the exchange of position, velocity and attitude data between the two satellites. The system consists of two patch antennas installed on opposite side panels of the satellites connected to the main S-Band Transceiver. The downlink data rate can be selected between 32 and 256kbit/s while the inter-satellite data rate is about 10kbit/s up to a separation distance of 5 Kilometers.

The two satellites carry identical payloads consisting of CNAPS – the Canadian Nanosatellite Advanced Propulsion System, a GPS receiver and a Formation Flying Control Unit.

The Canadian Nanosatellite Advanced Propulsion System uses heritage components from the NanoPS flown on CanX-2 in 2008. CNAPS will be used to perform precise maneuvers required to maintain a formation in the presence of orbital perturbations.

CNAPS is a gas propulsion system using liquid sulfur hexafluorid as a propellant that is ejected through four nozzles that are installed on one external panel of each satellite. The propulsion module measures 18 by 12.5 by 7 centimeters in size featuring two fuel tanks and four nozzles arranged in a cruciform pattern.

Each nozzle achieves a thrust of about 5 Millinewtons and the propulsion system reaches a specific impulse of around 35 seconds. The propellant is stored in a bottle that can hold 300 Milliliters which provides each satellite with a delta-v budget of 14 meters per second which is the limiting factor for the formation demonstration mission.

The four thrusters are pointing to the same direction – the thrust vector is controlled by pointing the satellite into the correct direction for each propulsive maneuver using the reaction wheels and attitude determination system. Each thruster can be controlled independently which is important for long propulsive maneuvers which will require independent control of the thrusters to avoid torques due to thruster misalignment.

A key component of the CanX-4/5 mission is the use of GPS data for absolute and relative navigation. Each satellite is equipped with a dual-band GPS antenna and a dual-band GPS receiver. The antennas are installed on a satellite side panel orthogonal to the thruster panel to be able to point close to zenith during propulsive maneuvers so that the maximum possible GPS satellites are within view.

The GPS receiver acquires precise position data for each spacecraft that is immediately transmitted via the Inter Satellite Comm Link for use by the other satellite’s Relative Navigation Software. This software uses the two sets of GPS data to derive a very precise relative position measurement provided the two satellites are using the same GPS satellites which is ensured by both satellites being in an identical orientation at all times with their GPS antennas facing the same direction. A minimum of four GPS satellites have to be in view to obtain a position fix, but six GPS satellites will significantly improve the accuracy of the navigation data. The navigation system of the satellites can tolerate short GPS blackouts, but will require additional fuel in such a scenario.

The GPS system can achieve an absolute position measurement at an accuracy of <5 meters and the velocity can be detected with <10cm/s accuracy. The relative navigation system using a common GPS set reaches an accuracy of under 5 centimeters in position and under 3 cm/s in velocity allowing extremely precise formation determination and control.

Image: University of Toronto SFL

Canadian Nanosatellite Advanced Propulsion System

Photo: University of Toronto SFL

Photo: University of Toronto SFL

GPS Receiver

The CanX-4 and 5 satellites were to launch firmly attached to one another using an Intersatellite Separation System for the separation once in orbit and fully commissioned to kick off the formation demonstration. However, the satellites will launch separated from each other and the separation system will not be put to use. The Intersatellite Separation System, ISS for short, consists of two nearly identical interfaces that are installed on the connecting side panels of the two satellites. ISS is comprised of a spring-loaded cone interface that builds the bonding surface between the two spacecraft coated in an electrically debonding agent that acts as a glue holding the satellites together.

For the separation, ISS applies a small voltage to the mechanism which causes the glue to weaken so that the loaded springs can overcome the adhesive force and push the two satellites apart. The two springs are loaded at a force of about 70 Newtons to impart a delta-v of about 8cm/s to each satellite at separation which is reduced to 2.6cm/s in the along-track direction by conducing the separation partially in the orbit-normal direction. This is done to set up the proper conditions for the first formation test.

Image: University of Toronto SFL

Image: University of Toronto SFL

The formation is controlled by FIONA, the Formation flying Integrated Onboard Nanosatellite Algorithm that uses a pre-programmed definition of each formation experiment to determine the tracking error of the deputy spacecraft and compute the optimal propulsion strategy to correct the error which is then commanded via the spacecraft controller.

Two formation schemes will be tested by the CanX-4 and 5 satellites: Along Track Orbit and Projected Circular Orbit formations. After recovery from the separation event, the two satellites will be assigned their roles – one will be the deputy spacecraft that performs all propulsive maneuvers while the other satellite does not use its propulsion system. Both satellites will exchange attitude data and the passive satellite will mimic the attitude of the deputy spacecraft to ensure both see the same set of GPS satellites for RelNav. The deputy satellite will initially conduct a drift recovery after the separation to get to a defined stationkeeping point for the initiation of the formation maneuvers.

For the Along Track Orbit formation, the two satellites will be in an identical orbit with one satellite flying ahead of the other at a defined separation distance that is maintained by propulsive maneuvers executed by the deputy satellite. The ATO scheme will be demonstrated at distances of 500 and 1,000 meters.

In the Projected Circular Orbit formation mode, the deputy enters an orbit with a slightly different inclination and eccentricity which, when observed from Earth, will seem like the deputy is orbiting the passive satellite. The PCO mode will be demonstrated at distances of 50 and 100 meters.

Each of the formations will be maintained for ten orbits with one orbit of reconfiguration maneuvers in between each formation mode or distance. The criteria for mission success are twofold – first, the level of accuracy to which the satellites can control their relative position in each formation will be assessed, and second, the ability of the deputy satellite to calculate the most efficient maneuvers will be studied to minimize fuel consumption.

Because the two satellites are identical, they can swap roles in the mission. Although the primary mission can be completed with one satellite acting as deputy and expending its fuel, a possible extended mission would be to use the previously passive satellite as deputy to repeat the formation exercise with different parameters. This mission design also adds redundancy in the event of a propulsion system failure on one satellite.

﻿AISat﻿

AISat is a prototype satellite to demonstrate a space-based AIS (Automatic Identification System) terminal for receiving signals from sea vessels to track ship movements on a global scale. Unlike other AIS spacecraft, AISat uses a helical high-gain antenna for the reception of AIS signals on highly frequented sea areas where conventional AIS terminals can not provide usable data due to signal overlap.

The Automatic Identification System is used by sea vessels that send and receive VHF messages containing identification, position, course and speed information to allow the monitoring of vessel movements and collision avoidance as well as alerting in the event of sudden speed changes. These signals can be transmitted from ship-to-ship and ship-to-shore to allow the monitoring of a local area, but deploying space-based AIS terminals allows a broad coverage and data relay to ground stations for monitoring of large sea areas. However, due to the large footprint of satellites, overlapping and signal collisions become a problem, especially for frequented traffic routes. AISat will demonstrate an approach that ensures the acquisition of usable data especially for highly frequented areas.

AISat is based on the Clavis satellite bus that itself is based on the PC/104 cubesat platform. The bus was developed by the German Aerospace Center to provide a cost-effective satellite platform that requires little man power for integration using commercially available CubeSat components. Clavis was designed to be able to host different payload types with high flexibility using a defined mechanical interface and a unified electrical and data interface.

The satellite bus uses a quad-stack configuration made up of the PC/104 cubesat boards with a back plane that connects the individual boards and also provides the data and electrical interface for the payload. Clavis uses body-mounted solar cells for power generation – five panels each consisting of 12 GaAs cells are installed on five side panels of the spacecraft to generate up to 15 Watts of power that is fed to a Power Conditioning and Distribution Unit which controls the state of charge of a 40Wh battery pack and distributes power to all satellite subsystems. The spacecraft uses 3.3V, 5V and 12V power buses. A switching board adds switching functionality to the power system and provides bus protection.

The spacecraft uses an ARM7 processor as the centerpiece of the Onboard Computer connected to the CAN and PC data buses as well as analog lines to control the attitude control system. The Onboard Computer includes 2MB of SRAM and 2x4MB of Flash Memory for avionics software and mission data. Attitude determination is accomplished through the use of the solar cells as sun sensors, a three-axis magnetometer and a three-axis gyro unit. AISat features three magnetic torquers for attitude actuation, but the satellite will be stabilized through a gravity-gradient generated once the large antenna is deployed.

The satellite uses a UHF communications system. A UHF transceiver and two monopole antennas with omni-directional coverage are used for telemetry downlink and command uplink.

AISat features a large helical antenna to take a new approach to AIS signal collection. Helical antennas are suited for transmitting and receiving circularly polarized signals and provide a high directional sensitivity. Their gain characteristics are excellent and increase with the number of windings. Using a high-gain antenna allows for a narrow aperture that only illuminates a very small footprint on the ground. The drawback of a helical antenna is the requirement of a conducting plate on its mounting interface acting as a reflector.

The AISat helical antenna measures four meters in length and 0.57m in diameter when deployed featuring a little over eight windings. It launches in a spring-loaded fashion that is released after spacecraft separation. The deployment is controlled by thin retracting cords that stop the antenna once it reaches its fully deployed position, preventing it from extending further due to the spring force. The antenna consists of a core using cylindrical sandwich struts and a fiber fabric hose to provide stability – the hose surrounds an inner core made of hard foam. The core is surrounded by thinned copper wires forming a metallic fabric tube. The control rods are electric isolators and ensure a stable self-deployment of the antenna within a reasonable timeframe.

Using the helical antenna, AISat can receive Class-A and Class-B AIS signals and the AIS Search and Rescue Transmitter signal of rescue craft and vessels in distress. AIS data is stored onboard and downlinked to ground stations for processing and distribution.

Image: German Aerospace Center

Image: German Aerospace Center

Image: German Aerospace Center

Photo: German Aerospace Center

Image: German Aerospace Center

Image: German Aerospace Center

﻿VELOX-1﻿

Image: Nanyang Technological University

Image: Nanyang Technological University

NanoSatellite

Image: Nanyang Technological University

PicoSatellite

Image: Nanyang Technological University

Spacecraft Structure

The VELOX-1 mission is comprised of the VELOX-1-NanoSatellite (NSAT) and the VELOX-1-PicoSatellite (PSAT) to demonstrate inter-satellite communications in orbit. Also, the project allows engineering students to participate in a multidisciplinary hands-on space project as VELOX is the first NanoSatellite of Singapore, designed and built at Nanyang Technological University. Secondary mission objectives include the acquisition of high-resolution Earth imagery and the demonstration of various payloads such as a vision system, a dual-field of view sun sensor and a quantum physics payload.

The VELOX NSAT is a 3U CubeSat that measures 10 by 10 by 34 centimeters with a total mass of 4.5 Kilograms that include the 250-gram PSAT which is just 6 by 7 by 3 centimeters in dimensions. The NanoSatellite consists of a Aluminum alloy chassis with load bearing parts made from stainless steel. The separation system, solar panel deployer and the optics extension system all use spring-loaded mechanisms for deployment after launch. NSAT features four deployable solar panels hosting Gallium-Arsenide Solar Cells delivering a peak power of 28.8 Watts. A 5,200mAh Li-Ion battery is used for power storage and a dedicated distribution unit delivers power to all satellite subsystems and payloads. Thermal control is ensured by multilayer insulation and heaters that maintain survival temperatures.

Attitude determination is provided by two Inertial Measurement Units, one dual-field of view sun-sensor and eight coarse sun sensors. Stabilization and three-axis control is accomplished by three Reaction Wheels with three magnetic torquers for momentum dumps. A GPS receiver is used for orbit determination. Dipole antennas are installed on the satellite for communications in the UHF and VHF bands reaching data rates of 9,600bps for downlink using BPSK modulation and 1,200bps for uplink that uses the AFSK protocol. NSAT is controlled by a main processor operating at 100MHz using a 2GB SD card for data storage and a UART and I²C data interface to connect to the various satellite systems.

The PSAT uses body-mounted solar cells to generate sufficient electrical power to operate the satellite controller and a communications system that will be used for a demonstration of inter-satellite communications. After launch and checkouts, the PSAT will be separated from NSAT by springs and start drifting away. Communication demonstrations will then begin to test out a space-to-space data link between the two satellites as the distance increases to determine the maximum possible range.

The Quantum Physics Payload installed on NSAT is 10 by 10 by 3 centimeters in size and weighs about 300 grams. The payload uses a laser diode to create photons, a logic circuit and a pair of photodiode detectors.

It uses miniaturized equipment to generate photon pairs to demonstrate quantum entanglement in orbit – a phenomenon that occurs when pairs or groups of particles interact in a way to that the quantum state of each particle can not be described independently, only the state of the system as a whole can be described. Quantum entanglement has been subject of many studies on the ground and in space, however many fundamental questions remain open. Flying quantum physics payloads on CubeSats allows many of such experiments with different parameters to be flown to gather a large data set on quantum entanglement in the space environment.

The second payload of the NSAT spacecraft is an optical imager that consists of an optics system, a CMOS sensor, an FPGA data acquisition board and a Payload Processing Unit. The optics unit features uses and extendable architecture – the required focal length is achieved by deploying the optics via a spring-loaded mechanism. The imager uses a 10.5 centimeter focal length that focuses the image into a radiation-hardened CMOS image sensor. Data from the sensor is retrieved via a field gate programmable array with 32MB RAM.

The VELOX-I NSAT has been built to operate for two years while PSAT will function for around one year which is sufficient for the comm demonstration mission.

June 29, 2014

Photo: ISRO

Preparations remain on track at India’s Satish Dhawan Space Center for the launch of a Polar Satellite Launch Vehicle on Monday at 4:22 UTC. PSLV is carrying the French SPOT-7 Earth Observation Satellite for orbit along with four small satellites – a pair of Canadian formation flying satellites, a German ship-tracking satellite and a satellite from Singapore to demonstrate an inter-satellite communications system.

Countdown operations started early in the morning on Saturday, 49 hours ahead of the planned launch time. The first day of the countdown was dedicated to propellant loading on the fourth stage and the Reaction Control Thruster Tanks of the first stage that were filled with Monomethylhydrazine and Mixed Oxides of Nitrogen. Also, final reconfigurations on the launcher were made as teams removed protective covers and closed the vehicle out for launch. Battery charging operations were in progress and testing of the various PSLV systems were conducted to ensure the launcher was ready for liftoff. Ground systems preparations also commenced to verify tracking and communication assets at the launch site and downrange were ready to support.

On Sunday, the Mobile Service Tower was rolled back to its launch position, exposing the PSLV rocket on the launch pad for final launch preparations. Propellant loading on the second stage of the PSLV commenced in the afternoon hours, local time, as the stage was filled with 40,700 Kilograms of Nitrogen Tetroxide and Unsymmetrical Dimethylhydrazine. Propellant loading operations wrapped up before midnight, local time, as the countdown entered its final hours.

Teams will evacuate the launch area for the final countdown operation that includes a set of testing operations on the electrical and flight control system of the PSLV that also receives its software for the 20-minute ascent mission. Over the final minutes of the countdown, the launcher pressurizes its propellant tanks for flight, transitions to internal power and enables its flight control system in launch mode for PS1 ignition when clocks hit zero.

In its Core Alone Version, PSLV only uses the regular four-stage stack without any Solid Rocket Boosters on the first stage. The rocket stands 44.5 meters tall, has a core diameter of 2.8 meters and weighs 229,000 Kilograms at liftoff using a combination of liquid- and solid-fueled rocket stages. The PSLV launcher consists of a large core stage that is 20.34 meters long and holds 138,000 Kilograms of solid propellant – making it one of the largest solid rocket stages ever flown. It provides a whopping thrust of 495,600 Kilograms.

The second stage of the launch vehicle uses storable propellants that are consumed by a single Vikas 4 engine that provides 81,500kg of vacuum thrust. The stage is 12.8 meters long featuring a 40,700-Kilogram propellant load. The PS3 stage of the PSLV launcher is solid-fueled, being 2.02 meters in diameter and 3.54 meters long holding 6,700 Kilograms of HTPB-based propellant. The third stage provides a total thrust of 24,900 Kilograms. Stacked atop the third stage is the PS4 Upper Stage that again uses hypergolic propellants – Monomethylhydrazine fuel and Mixed Oxides of Nitrogen – consumed by two L-2-5 engines. The stage is 2.02 meters in diameter and 2.6 meters long featuring a partial fuel load of 1,600 Kilograms. Upper stage thrust is 1,500 Kilogram-force.

Three seconds before T-0, the Roll Control Thrusters at the base of the first stage are started before the Solid Rocket Motor is ignited at the moment of T-0.

Photo: ISRO

Blasting off at a thrust to weight ratio of 2.0, PSLV completes a short vertical ascent before starting a Pitch and Roll maneuver to align itself with a pre-planned ascent trajectory taking it south across the Indian Ocean.

This mission is targeting a polar orbit which requires PSLV to implement a Dogleg Maneuver - a guided, powered turn during the ascent phase of the mission. The maneuver comes at the cost of ascent capability, but range safety requirements dictate that the vehicle can not fly over inhabited areas on the island of Sri Lanka. Initially, PSLV will head east before beginning to turn. When the launcher is clear of the island, it starts flying south-west to reach its desired orbital inclination.

Throughout PS1 burn, three axis control is provided by a Secondary Injection Thrust Vector Control (SITVC) for yaw and pitch and two radially mounted thrusters for roll.

Image: ISRO

When the first stage has burned out, it separates from the second stage at T+1:50 followed by PS2 ignition an instant later. Staging occurs at approximately 53 Kilometers in altitude while the vehicle is traveling 1.5km/s. During the second stage burn, the launch vehicle departs the dense atmosphere – allowing the vehicle to jettison its payload fairing at T+3:08 at an altitude of 131.5 Kilometers, exposing the spacecraft for the remainder of the ascent as aerodynamic forces can no longer damage the vehicle. Control during second stage flight is provided by engine gimbaling for pitch and yaw and a roll reaction control system.

The second stage burns for about two minutes and 33 seconds before separating from the third stage that then ignites and assumes control of the flight at T+4:23. Stage Separation occurs 219 Kilometers in altitude with the vehicle moving at 3.57km/s. The solid-fueled third stage burns for 112 seconds to boost the stack to a sub-orbital trajectory. It uses the fourth stage Reaction Control System for three-axis control. After burnout of the PS3 stage, the stack begins a coast phase – initially holding onto the spent third stage before separating it at T+8:41 and continuing to coast for ten more seconds for the ignition of the fourth stage at T+8 minutes and 51 seconds.

The coast ahead of fourth stage ignition is implemented to allow the vehicle to reach a position near the apogee of its trajectory so that the fourth stage burn can serve as a circularization maneuver, raising the perigee to nearly match the apogee of the orbit.

Once the stack reaches its desired altitude, the two L-2-5 engines of the fourth stage ignite at T+8:51 on a burn of about 7 minutes and 26 seconds to boost the stack into its target orbit of 655 by 658 Kilometers at an inclination of 98.2 degrees.

Following orbital insertion, the fourth stage re-orients to begin the spacecraft separation sequence. SPOT-7 will be released at T+17 minutes and 53 seconds, followed by the AISat spacecraft at T+18:33. The two Canadian satellites will be sent on their way at T+19:03 and T+19:33. Finally, VELOX-1 from Singapore is deployed at T+19:58 to conclude the PSLV mission.

Countdown underway for India's PSLV set to Launch on Monday

June 28, 2014

The Indian Space Research Organization has initiated the launch countdown of its Polar Satellite Launch Vehicle set for liftoff on Monday to deliver the SPOT-7 Earth observation satellite to orbit along with four secondary payloads. Blastoff from the Satish Dhawan Space Center is set for 4:22 UTC on Monday as PSLV will embark on a twenty-minute ascent mission headed for Sun-Synchronous Orbit.

To lift SPOT-7 and the four small satellites, PSLV can fly in its most basic version known as Core Alone configuration that does not use any Solid Rocket Boosters and only consists of the four-stage PSLV stack. PSLV-CA stands 44.5 meters tall, has a core diameter of 2.8 meters and weighs 229,000 Kilograms at liftoff using a combination of liquid- and solid-fueled rocket stages. The first stage, PS1, is one of the largest solid rocket motors in the world with a mass of 168 metric tons and a thrust output of 4,860 Kilonewtons. Using 41 metric tons of hypergolic propellants, the liquid-fueled second stage sports one 799kN Vikas engine. The third stage of the launcher delivers 244kN of thrust using solid propellant while the fourth stage of the PSLV burns storable propellants using two L-2-5 low-thrust engines. In the CA version, the four stage launches with a reduced fuel load of 1,600kg.

Preparations for the C23 mission started with the assembly of the launch vehicle that began with the installation of the PS1 stage on the launch table followed by the installation of the liquid-fueled second stage, the solid-fueled third stage and the upper stage of the launch vehicle. With PSLV assembly complete, the rocket was ready to go through a long testing campaign to verify the launcher is ready to execute its mission.

The SPOT-7 satellite and the secondary payloads began their final processing flow after delivery to the launch site.

Photo: ISRO

SPOT-7 went through a series of testing operations to verify the satellite was in readiness for launch. As part of hazardous processing, SPOT-7 was loaded with hydrazine monopropellant for orbit adjustments during the operational mission. In mid-June, the SPOT-7 satellite was installed atop the PSLV rocket via its payload adapter. The four secondary satellites, CanX- 4 and 5, AISat and VELOX-1, were installed in their deployment mechanisms on the adapter ring of the fourth stage to be deployed after the separation of SPOT-7. In addition, PSLV-C23 hosts one attached payload, the Advanced Inertial Navigation System which will ride uphill with the launcher, making navigation measurements during ascent that are purely used for the validation of the system for potential use on Indian launch vehicles in the future. Data provided by AINS is not used to navigate the PSLV launcher on this flight and is collected for downlink to the ground to examine the accuracy and performance of the new navigation system that is smaller in size than the old unit, but delivers a higher accuracy.

Photo: ISRO

Photo: ISRO

After the installation of the payloads, the protective payload fairing was installed around the stack to complete the integration process. Next, the launch vehicle was put through extensive testing of all subsystems to confirm the vehicle is ready for flight. Global checks of the assembled launch vehicle were performed to make sure all electrical and data connections were functional. Testing included countdown and launch rehearsals as the launch team went through the terminal phase of the countdown and performed a flight simulation to ensure that all subsystems of the launcher are functioning properly and the computers issue the appropriate commands at the correct times. With the rocket standing tall on its launch pad, teams performed final pre-countdown activities to prepare the launch vehicle and ground systems for the initiation of the long launch countdown.

The 49-hour launch countdown was initiated early on Saturday to start the methodical process of configuring the launch vehicle for flight. Countdown operations include Propellant Loading on the liquid-fueled stages and extensive checks of the rocket and the spacecraft are conducted as well as battery charging. In addition to that, India’s Ground Network of Tracking Stations is being configured for the flight.

After the countdown was initiated, teams started loading loading the fourth stage with Monomethylhydrazine fuel and the Reaction Control Thruster Tanks on the first stage were also loaded with MMH. Next, the fourth stage’s tank was filled with MON-3 oxidizer (Mixed Oxides of Nitrogen) as were the RCT tanks on the first stage. Loading of the second stage with 40,700 Kilograms of Unsymmetrical Dimethylhydrazine and Nitrogen Tetroxide will be underway on Sunday.

Over the course of the final countdown hours late on Sunday and into the early hours on Monday, teams perform final hands-on work to close out the launch vehicle and launch pad facilities while the launch team monitors all systems of the launcher and the spacecraft, putting the vehicle through a last set of electrical, communications and control systems checks before pressing into the final countdown sequence. During the final minutes of the countdown, the launcher pressurizes its propellant tanks for flight, transitions to internal power and enables its flight control system in launch mode for PS1 ignition when clocks hit zero.

Photo: ISRO

Photo: ISRO

Photo: ISRO

Photo: ISRO

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